Morphology of cycas:
Cycas plant looks like a beautiful ever green palm. The plant of cycas is sporophyte (2n) and bears root, stem and leaf. Mature plants are usually 8 to 14 feet in height.
Sporophytic life cycle of cycas:
Plant body of cycas is sporophytic. Cycas plant has two distinct generations sporophytic and gametophytic. Sporophyte sexually reproduces by means of spores.
Sexual Reproduction: Plants are dioecious, sporangia are borne on sporophylls which group together to form the cones or strobili. Cones of cycas are terminal on the ste,.
Male reproductive organs or Male cone or Staminate strobilus:
Male cones of cycas occur singly in the center of crown. It is woody, wedge shaped, blunt ended about 50cm logn. Male come of cycas is among the largest reproductive structures in the Gymnosperms.
Each male cone has an axis on which the sporophylls are arranged spirally. Sporophyll is known as microsporophylls or stamens, each of which is flattened, narrows below and expanded above into sterile disc the apophysis. Microsporangia (pollen sacs) are borne in soral groups of three to six known microsori. They dehisce by a slit which extends radially from centre of rows. Microsporangium is a globular or ovate body with several layered wall. It contains numerous microspores or pollin grains.
Development of microsporangium and microspores:
Microsporangium develops from a single hypodermal cell of microsporophyll. The initial cell divides periclinically into an outer sterile or primary cell and inner primary sporagenous cell archegonium. Primary wall cell divides by repeated periclinical and anticlinical divisions and a sporangium wall several cells in thicknss are developed which is covered over by epidermis. Primary sporgenous cell divides and forms large mass of sporagenous cells or spore mother cells.
From periphery of sporogeous tissue the tapetum is differentiated. As development proeeds the sporogenous cell round off and becomes isolated as spore mother cells which begin to float in a nutritive medium formed by disorganisaition of tapetum. Spore mother cells divide by reduction division (Meiosis) and followed by simle division so as to form four microspores or pollen grains. Thus microspores get developed which are haploid and regarded as first cell of male gametophyte.
Female reproductive organs or Female cone or Oviclate strobilus:
Genus cycas is peculiar in its ovulate strobilus as it is not teme compact cone strobilus but simply a rosette of megasporophylls arising spirally in acropetal succession and loosely arranged on the stem in a rosete, the ordinary crown of foliage leaves around the stem tip. The apical meristem remains unaffected and continues to grow so as to form future leaves and sporophylls. Below the whorls of foliage leaves are whorls of scale leaves. Thus it is evident that the megasporophylls replace scale leaves and are morphologically equivalent to them.
Megasporophylls of cycas are large and show that they are leafy in naure. Each megasporophyll is brownish palmate with few ovals or megasporangium on the stalk. Ovules of cycyas are the largest in plant kingdom.
Megasporangium or Ovule:
Each ovule of cycas has single massive integument which surrounds the mass of parenchymatous tissue the nucleus around except at the upper end where a narrow passafe way the microphyle is formed. Main body of Ovule is micellus or Megasporangium proper. The integuments in a well developed ovule consist of three recognised layers, an outer and inner flashy layer of simple strucute and very complex stone layer between them. The vascular supply of ovule is practically uniform.
The vascular strands are composed of mesrach xylem. At the apex of micellus is nucell or beak which is forced up into the micropyle. Centre of beak becomes hollow to form the pollen chamber in which pollen grains or microspores collect after pollination. Pollen chamber unites with microphyle. Deep within the nucellar tissue is single lage embryosac.
Development of Ovule and Megaspore cell:
The ovule arises as a hypodermal mass of meristematic cells on the megasprophyll edges. Cells divide rapidly and form the nucellus. These cells are also activated and growing rapidly invest the from all sides. A ring like intengument arises from the base of nucellus and surrounds it alroud except at the top which is micropyle. Now deep within the nucellar tissue a cell bigger in size with clear nucleus is present known as megaspore mother cell which divides twice in the usual way, the first division is reduction division and second is ordinary divison to form a linear tetrad of megaspore. Thus four megaspores so formed are haploid in nature and are first cells of female gametophyte (embryosac, endosperms and archegonia).
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